Estuaries and Coasts

, Volume 37, Issue 6, pp 1572–1581

Transient small boats as a long-distance coastal vector for dispersal of biofouling organisms


    • Marine Invasions LaboratorySmithsonian Environmental Research Center
  • Ian Davidson
    • Aquatic Bioinvasion Research and Policy Institute, Environmental Science and ManagementPortland State University & Smithsonian Environmental Research Center
  • Gregory Ruiz
    • Marine Invasions LaboratorySmithsonian Environmental Research Center

DOI: 10.1007/s12237-014-9782-9

Cite this article as:
Ashton, G., Davidson, I. & Ruiz, G. Estuaries and Coasts (2014) 37: 1572. doi:10.1007/s12237-014-9782-9


Alaska is at the northern end of an apparent latitudinal trend of decreasing coastal marine introductions on the West Coast of North America. Historical propagule supply may have played a role in forming this trend, but few studies have evaluated propagule supply to northern latitudes. Here, we examined the role of small boat traffic as a mechanism of long-distance spread for nonindigenous species (NIS) into coastal Alaska. We used a combination of public records, marina surveys, and boater interviews to characterize vessel traffic patterns and boater behaviors. In-water SCUBA sampling of recently arrived transient boats provided data on extent, richness, composition, and biogeography of biofouling incursions to Alaska from outside of the state. We documented a striking seasonality and directionality of vessel traffic, and most vessels were on voyages of >900 km. Most transient vessels sampled had few organisms, although one third had >100 organisms on submerged surfaces. Several NIS were recorded, including two that are not known to be established in Alaska (Watersipora subtorquata and Amphibalanus improvisus). The seasonal northward pulse of vessels and their cumulative biofouling species represent an important incursion mechanism for species yet to establish at the northern edge of a marine bioinvasion front in the northeastern Pacific. The low numbers of NIS sampled in this study coincide with the low number of marine NIS known from Alaska, which suggests that an opportunity remains to promote awareness and management of the vector to limit NIS influx to the region. This may be particularly relevant for future scenarios of increased vessel traffic and ocean warming, which are likely to interact to increase establishment success of invaders from the south.


Anthropogenic transportBiofoulingIntroducedLatitudinal gradientNonindigenous speciesVector


There is an apparent decline in introductions of terrestrial and freshwater nonindigenous species (NIS) with latitude (Sax 2001; McKinney 2006). The trend has also been recorded for marine invertebrates on the West Coast of North America (Ruiz and Hewitt 2009). Recent records show a stark contrast in invasion histories between the southern and northern ends of the northeastern Pacific coastline with 257 established NIS recorded in California but only ten in Alaska (Ruiz et al. 2011). Following their initial detection in the south of this range, populations of many NIS have subsequently been recorded further north, suggesting a general trend of northward spread by NIS (Fig. 1; Ruiz et al. 2011). All ten of the NIS described in Alaska were reported previously from a bay further south, and a more recent Alaskan invader, Didemnum vexillum, also has established populations in states and provinces to the south (Cohen et al. 2011). Furthermore, several additional southern NIS that have not yet been recorded in Alaska are capable of establishing themselves should founder populations make the initial incursion there (deRivera et al. 2006, 2011; McClory and Gotthardt 2008; Zabin et al. 2009). This trend, combined with predicted ocean warming and increases in maritime vector activity, suggests that the rate of invasion at high latitudes is likely to increase (Ruiz and Hewitt 2009).
Fig. 1

Latitudinal trend of introductions of marine NIS to the Pacific coast of North America. Pie charts show the ratio of species first detected (introduced or native) in that state/province (gray), those detected earlier in the south (black), and those detected previously in the north (white). Size of the pies illustrates relative number of marine NIS established in each region (actual numbers in parentheses). Note that California was the southern limit for these data, and therefore introductions to the south would not be possible here. Data synthesized from Ruiz et al. (2011). Only species confirmed as introduced were included in the dataset. Location of the study site, Ketchikan, AK, is also indicated

Vessel biofouling is an important vector responsible for the initial incursion of established NIS on the US West Coast (Fofonoff et al. 2003; Ruiz et al. 2011). The vector has a long history of transferring marine species (Carlton 1985) and has operated on a global scale to heavily influence invasion patterns (Hewitt and Campbell 2009). Biofouling of commercial vessels has been the subject of most studies to date (Coutts and Taylor 2004; Davidson et al. 2009), but small vessels are also significant vectors of NIS transfers, including both coastal and long-distance transport of marine organisms (Floerl and Inglis 2005; Ashton et al. 2006; Minchin et al. 2006). When present, fouling communities on small boats can include diverse assemblages of marine plants and animals, including mobile taxa (Floerl et al. 2005a; Farrapeira et al. 2007; Darbyson et al. 2009; Davidson et al. 2010). Small boats offer opportunities for biofouling accumulation that differ from in-service commercial ships because they have divergent maintenance routines and tend to have long port durations (>24 h and up to many months in most cases) in sheltered marinas that are ideal environments for vessel colonization (Floerl and Inglis 2003; Minchin et al. 2006). Small boats also follow different voyage routes and visit different harbors compared with commercial ships, although they do overlap.

Davidson et al. (2010) previously noted the importance of recreational boats for spreading NIS on the US West Coast, especially at sites of high-vector activity (vector hubs) that are also hotspots for invasions (e.g., San Francisco Bay). Small boats may play an important role among vectors to northern latitudes of the northeastern Pacific, where both fishing grounds and tourism destinations attract vessels from invaded southern latitudes. In fact, vessel biofouling is associated with the initial incursion of eight of the ten NIS already established in Alaska (Ruiz et al. 2011). Despite this historical vector strength, no study, to date, has evaluated small vessel traffic arriving to Alaska or sampled the biota associated with that traffic. While recent studies have documented NIS on small vessels in British Columbia and California (Davidson et al. 2010; Clarke Murray et al. 2011), as well as other global regions (Minchin et al. 2006; Ashton et al. 2006), these analyses have focused primarily on vessels in their home marinas and their potential to transfer species to other locations. Even when the voyage histories of such vessels are known, authors must infer that the vessels have carried the associated biofouling organisms to recent destinations (Clarke Murray et al. 2011). This is problematic because there can be no certainty about the point at which species colonized a vessel (i.e., before or after recent voyages) or, if present, whether they will survive the transit to future destinations. In contrast, analyses of other vectors usually have assessed the actual transport or arrival of NIS (e.g., ballast sampled during voyages, Lavoie et al. 1999; bait boxes examined at point of sale, Haska et al. 2011; commercial vessel hulls studied on arrival to a port, Davidson et al. 2009), providing a direct measure of species transfer.

Here, we characterized transient small vessel traffic patterns and biofouling communities of those vessels to assess this mechanism of introductions to high latitudes of the northeast Pacific. The biofouling organisms on all transient vessels were surveyed within 24 h of their arrival to Ketchikan, AK, providing confidence that associated sessile organisms had arrived with the vessel from a previous port of call rather than locally sourced invertebrates. By assessing the status of the organisms within 12 h of collection, we also determined the condition of species upon arrival (viable live organisms versus moribund versus dead). Ketchikan was chosen as a case study location because it is a key maritime gateway into the state (McGee et al. 2006) and an important port of call for both recreational and fishing vessels arriving from western North America. The specific goals of this study were to (i) estimate spatial (voyage origins and routes) and temporal (seasonal) transient small vessel movement patterns to Ketchikan, (ii) characterize hull husbandry strategies employed by owners of transient vessels, and (iii) assess the extent and composition of biofouling associated with the hulls and underwater surfaces of transient vessels.


Vessel flux

Ketchikan is situated in SE Alaska (55°21′00″N, 131°40′24″W, Fig. 1) and is the first coastal city and port encountered by vessels arriving from British Columbia or further south. As a result, this is an entry point for many vessels travelling to the state, including commercial barges, cruise ships, and small boats (McGee et al. 2006).

To characterize the number and source of small vessels, data from both Customs and Border Protection (CBP) and Ketchikan Ports and Harbors (KPH) were analyzed. CBP requires registration by every arrival from a foreign port; registration includes the last port of call, next port of call, and vessel type and length. KPH maintains a record of daily moorage rates; the record includes the date, an anonymous vessel identifier code, and the zip code that the owners listed during registration. Both CBP and KPH record arrivals on a daily basis, and the number of arrivals per month throughout 2009 was used to assess the temporal trend (seasonality) of small vessel arrivals.

CBP does not collect information on transient vessels registered in Alaska, and KPH records exclude those booking moorages in Ketchikan for longer periods (1 month or more). Thus, our data underestimated the numbers of actual arrivals, but they capture a majority of transient vessels for characterizing patterns of arrival to the area.

Hull husbandry

During May 2009 and 2010, operators of transient boats were interviewed upon arrival to Ketchikan public docks. Information regarding the vessel (type and length), hull husbandry (date and type of antifouling paint application and in-water cleaning schedule), and the ports of call on the voyage to and from Ketchikan (arrival route and planned destinations) was solicited. Answers relating to hull maintenance practices and voyage history from the questionnaires were tabulated and used to characterize husbandry patterns among boaters arriving to Ketchikan.

Biofouling extent and composition

Fifty transient small vessels were sampled for hull fouling organisms during May of 2009 and 2010. Vessels were sampled as soon as possible after their arrival into Ketchikan (within 24–72 h), and all vessels had arrived from outside of Alaska. Sampling involved three SCUBA divers. One took photoquadrats using a Canon Powershot A650 camera and housing with an affixed PVC frame with quadrat dimensions of 22 × 16 cm. The other divers made collections of organisms (see below) from areas that had just been photographed. Images were used to estimate organism abundance, and careful collections were made to identify biofouling community composition. All submerged surfaces of the vessels were inspected, including the hull, rudder, propeller, struts, shafts, keel, bilge keels, intake pipes, thrusters, grates, cooling systems, knot meters, anodes, and stern platforms. Samples and images were collected from all locations where organisms were encountered.

Estimates of invertebrate abundance (numbers of individuals or colonies) were made by analyzing the photographs of all underwater hull locations where macroinvertebrates were detected. For areas with high densities of organisms (>400 individuals per image), abundance per photoquadrat was estimated by counting the organisms in six haphazardly selected subquadrats and multiplying by the area of the image. The influence of vessel type (sailing, motor, or fishing) as a factor determining fouling extent was tested using Bray–Curtis similarity measures calculated in PRIMER (PRIMER-E Ltd, Plymouth). For low-density areas, all invertebrates were enumerated (subsampling was not necessary). For sampled vessels, we had a corresponding interview or questionnaire response sheet. Correlations between duration since last antifouling treatment and abundance of fouling organisms were tested using Spearman’s rank correlation. Abundance from individual hull locations (e.g., rudder and keel) was tested independently as well as the combined total abundance across the entire submerged vessel surfaces.

On vessels with limited biofouling, all organisms encountered were collected. On highly fouled vessels, multiple samples of biofouling from each hull location (rudder, propeller, struts, thrusters, hull, etc.) were collected. Our primary goal was to collect and identify the maximum number of species present. Usually within a few hours after collection (and in some cases overnight), samples were analyzed in the laboratory, and organisms were initially identified to morphotaxa level, scored for live–moribund–dead status, and preserved for species identification and verification. The identity of voucher species was confirmed by taxonomic experts or through genetic sequencing of the CO1 gene and comparison with sequences in GenBank ( For those taxa identified to genus or species level, biogeographic status in regions in the northeastern Pacific (native, introduced, and cryptogenic) was determined using literature searches. For those species that were considered introduced to the northeastern Pacific (SE Alaska specifically), the locations of previous records of established populations were used to assess the direction of (assumed) spread along this coastline.


Vessel flux

There were records of 678 private (noncommercial) boat arrivals to Ketchikan from CBP, including 537 arrivals by 516 recreational vessels and 141 arrivals by 81 fishing vessels (registered as “Private” or “Fishing,” respectively, by the CBP). Independent data from KPH support an annual flux of this magnitude, with 458 out-of-state vessels registered for daily rates at Ketchikan facilities during 2009 (excluding vessels registering for a 1- or 3-month permit). There was a strong seasonal signal to the arrivals (Fig. 2). Private recreational vessel arrivals occurred predominantly (>99 %) from April to October, with a peak in late June. Most private fishing vessels arrived from late March to September, with occasional records from other months, and a much less pronounced seasonal peak compared with recreational vessels (Fig. 2).
Fig. 2

Monthly arrivals of small vessels registered with CBP (fishing and private vessels shown separately) and those registered for the daily rate with KPH (all small vessels)

The vast majority of transient vessels arriving to Ketchikan had come from regions to the south (Fig. 3). CBP data indicated that 95 % had the last port of call to the south, before entering Alaska, and 58 % listed Prince Rupert as the last port (Fig. 3a). Both questionnaire and KPH data suggested Washington State as an important origin for many vessels travelling to SE Alaska (Fig. 3b). Eighty-one percent of vessel owners had home addresses in California, Oregon, Washington, and southern British Columbia. The origin of boaters with addresses outside of the West Coast (19 %) could not be assigned with any confidence, and the directionality of these vessels was not assessed.
Fig. 3

Voyage history of vessels arriving to Ketchikan in 2009. Data show a location of last port of call for private vessel arrivals registered by CBP (black filled, n = 652) and b home address for out-of-state customers of KPH registering for daily moorage in 2009 (white filled, n = 374) and home address for boat owners surveyed in person in Ketchikan (gray filled, n = 55). AK (N) and AK (S) indicate locations in Alaska to the north and south of Ketchikan, respectively. Other/Unknown indicates CBP records that did not include a last port of call or KPH records with home addresses in states outside of the northeastern Pacific

Eighty percent of owners surveyed on their arrival to Ketchikan had stopped at another port of call during their transit to Alaska, and all respondents were travelling further north into Alaska. Based on the KPH data, 58 % of the vessels (n = 274) registered more than once, most likely travelling further into Alaska before returning south via Ketchikan. An additional 198 vessels (42 %) only registered once to KPH, suggesting that either Ketchikan was their furthest destination north or the vessel did not stop in Ketchikan on the return voyage south.

Hull husbandry

The questionnaire responses indicated that most vessels either clean or paint their hull prior to travelling to Ketchikan. From the 68 transient respondents (~10 % of transients expected in 1 year), 53 included answers to the hull-husbandry questions. Twenty-one vessels (40 % of the 53 respondents) had been painted within 2 months prior to their voyage to Ketchikan. A further 50 % of vessels were cleaned within 2 months prior to their arrival (although their paint was older than 2 months and sometimes over 2 years old). Only two vessels reported paint older than 12 months that had not been cleaned within the previous 2 months.

Biofouling extent and composition

Among the 50 transient vessels sampled, 38 % were found to have no detectable macrofauna or macroalgae (Fig. 4). Biofouling extent (abundance of individuals and colonies) was quite variable on the remaining 31 boats that had marine biofouling, ranging from six to greater than 21,500 organisms per boat. Vessel type (sail, motor, or fishing) did not explain differences in either raw or categorical abundance (average similarities within each vessel type were between 15 and 19 %). Fourteen vessels (28 % of the total) had tens of organisms or fewer, but we recorded thousands of organisms on hulls and underwater surfaces of eight vessels (16 %). The high abundance of invertebrates described for some vessels was due to an apparent recent mass settlement of barnacle cyprids (e.g., boats 34 and 38 in Fig. 4 for which there were no maintenance data), while others simply had very well developed assemblages of organisms. Propeller shafts were the vessel locations where biofouling was most frequently recorded (18 of 50 vessels), but the highest numbers of organisms were recorded on vessel keels. In contrast, hull surfaces at the bow were least often covered with biofouling, and when present, the fewest organisms were recorded from bow thrusters.
Fig. 4

Color gradient plot of invertebrate abundance per hull location on recreational vessels (n = 50). Transient vessels were sampled within 72 h of arrival unless marked with an x (arrival date unknown) or shown in italics (arrived 6 months prior to sampling). With the exception of the 14 vessels listed at the top, boats are ranked in order from the most recently cleaned/painted (top) to those with the longest duration since hull maintenance (bottom). Solid horizontal lines divide categories of time since most recent hull-husbandry action; crossed abundance indicates that the vessel did not have a bow thruster. Spearman’s rank correlation coefficients between time since last cleaning and abundance of fouling organisms are indicated in the legend (ρ)

Barnacles, bryozoans, and hydroids were the most prevalent organisms among the taxa recorded (see electronic supplement for the complete species list). Unattached mobile organisms were also encountered quite often, primarily amphipods, isopods, and polychaetes, whereas soft-bodied forms (ascidians, anemones, and sponges) were only recorded occasionally (Fig. 5). More than half of the taxa that were identified to species level following collection from transient vessels were considered native to Ketchikan (n = 24; 67 %; electronic supplement), although most records probably represent transfers from distant populations (outside of Alaska) for these species. Six species have been described from Alaska previously as either introduced (Table 1, Botrylloides violaceus, Botryllus schlosseri, Caprella mutica, and Schizoporella japonica) or cryptogenic (Celleporella hyalina and Cryptosula pallasiana). A further six taxa have not been reported from Alaska previously but have been described from states to the south as either native (Celleporaria brunnea), introduced (Bugula stolonifera, Watersipora subtorquata, Diplosoma listerianum, Amphibalanus improvisus), or cryptogenic (Bugula neritina). Of those that have not been reported from Alaska previously, A. improvisus, C. brunnea, and W. subtorquata were alive upon collection. All eight of the NIS that were recorded on boats and are considered introduced to the northeastern Pacific have shown range expansions (subsequent records) in the northward direction (Table 1).
Fig. 5

The prevalence of taxonomic groups on the hulls and stern appendages of private transient vessels sampled in Ketchikan. The occurrence of 13 biofouling taxa (functional groups) was plotted as a percentage of the total number of vessels surveyed using SCUBA. Filled shade indicates the status of the organisms as live (black) or dead (white)

Table 1

Status of species which are not native to Alaska and were identified from recreational boats sampled in Ketchikan in 2009 and 2010

Status on the West Coast


Direction of records

Described previously in Alaska



See electronic supplement; n = 24



Botrylloides violaceus


Botryllus schlosseria,b


Caprella mutica


Schizoporella japonica



Celleporella hyalina


Cryptosula pallasiana


Not described previously from Alaska


 Native southern species

Celleporaria brunneab



Bugula stoloniferaa,b


Watersipora subtorquatab


Diplosoma listerianuma,b


Amphibalanus improvisusb



Bugula neritinaa,b


Species are divided into those that have been previously described in Alaska (top six species) and those for which this is the first record (bottom six species). The status is divided further into native, introduced, and cryptogenic. Where applicable, the direction of subsequent records on the US West Coast have been noted in the right column

N northward, N/A spread of established populations of this species have not been documented

aOnly dead individuals were found

bNot previously recorded from Ketchikan

For vessels with both biota and questionnaire data, the only significant correlation with duration since most recent hull painting or cleaning was found for abundance of fouling organisms on the aft of vessels (Spearman’s rho ρ = 0.366, p = 0.03). All other relationships, including abundance and diversity for the whole vessel, were weak and not significant (all ρ’s < 0.30, p > 0.10).


A strong latitudinal gradient exists for NIS richness in coastal marine habitats of the northeastern Pacific, from California to Alaska, with relatively few NIS established currently in Alaska. Propagule supply is considered generally to be an important driver of invasion patterns (Lockwood et al. 2005) and is thought to be a major historical factor in the observed latitudinal pattern (Ruiz et al. 2011). While past analyses have focused primarily on commercial shipping as a source of propagules to Alaska, this study is the first to characterize the small boat biofouling vector (including both the traffic patterns and associated biota) to the region and consider its potential significance for long-distance northward dispersal of NIS to Alaska. Our direct measures of vessel flux and associated biota also differ from many other studies of small boats in other regions which have generally described the presence of known NIS, those already established in an area, on boat hulls sampled in the same area (e.g., Johnson et al. 2001; Ashton et al. 2006; Mineur et al. 2007; Davidson et al. 2010; Clarke Murray et al. 2011).

Our results show that private vessel arrivals to Alaska have both a strong seasonality and directionality, which may affect the likelihood of NIS transfer. Many small privately owned vessels use Ketchikan as their first port of call in Alaska after travelling the “inside passage.” More than 650 small private vessels transited Ketchikan in 2009, and 85–90 % of these arrived during May–August (summer months). The punctuated, seasonal pattern of arrivals to Ketchikan is biologically significant because the transit period coincides with the peak spawning time for many types of marine invertebrates, including organisms in the biofouling community (Reitzel et al. 2004) and may increase the opportunity for colonization of vessels by biota. Seasonal shifts in small boat traffic have also been noted for vessels on the Atlantic coast of North America, in the Mediterranean, and in Hawaii (Osman unpublished; Minchin et al. 2006). This is in contrast to commercial vessels, for which the routes and purpose are less contingent upon season.

The directionality and previous ports of call indicated that the summer peak of vessel arrivals was dominated by a northward movement of vessels from ports in Washington State and southern British Columbia. While most vessels indicated Prince Rupert as the last port of call, interviews with boaters and analyses of harbor master customer information revealed that the vast majority of arrivals originated in Washington, Oregon, California, and southern British Columbia. The strong directionality of arrivals from southern bays and estuaries is biologically significant because many of these southern locations are potent sources for NIS transfers, having large numbers of established populations (NEMESIS 2013). In addition, the fact that many vessels travel the same route creates strong links (connectivity) with southern bays that can result in repeated inoculations of the same species, an important factor that may promote persistence and establishment of founder populations of NIS (Padilla et al. 1996; Ruiz et al. 2000; Johnson et al. 2001).

Many small vessels stop in multiple bays on their voyages north, which provides the opportunity for stepwise introductions along the coast, with species from California being first introduced to Oregon and/or Washington and subsequently throughout British Columbia and Alaska, not necessarily all via one vessel/voyage. The geographic spread of many NIS also suggests this stepwise movement to the north, following the initial colonization of the northeastern Pacific (Fig. 1; Tepolt et al. 2009; Pilgrim and Darling 2010). It is likely that small vessels contribute to this pattern of spread, for some of the biofouling organisms, but the relative contribution remains unknown. Such a long-distance pulse of small vessels that follow a directional seasonal pattern may be much more common for continental coastlines, while the source directions of arrivals to island populations are more variable (Floerl et al. 2005a; Godwin et al. 2004). In contrast, resident vessels in Ketchikan generally do not travel more than 100 miles, visiting local bays and harbors for short durations (Ashton, unpublished data). These vessels have the potential to spread NIS beyond founding populations in marinas, including remote locations (Wasson et al. 2001). This may create a “hub-and-spoke” effect (Carlton 1996), with Ketchikan acting as the hub and downstream marinas and bays in Alaska becoming the spoke nodes. Thus, while remote marine habitats and small recreational harbors are never visited by commercial vessels, their position downstream of a pivotal small boat gateway port can make them susceptible to NIS incursions.

In our survey of 50 transient vessels, we detected five species that are nonnative to the northeastern Pacific and were alive upon collection; dead individuals of an additional three nonnative species were also collected. This is a considerable number when compared with the total of ten marine NIS known in Alaska. Clarke Murray et al. (2011) found nine nonnative or cryptogenic species on vessel hulls sampled in British Columbia, Canada. While this is more than found in the current study, it is less than 20 % of the known marine NIS in British Columbia (n = 62; Ruiz et al. 2011) and all were sampled from resident vessels, albeit several of the vessels were known to travel to other locations. Transit along the Pacific coastline into Alaska, using the inside passage with stops en route, is likely to provide sheltered conditions, possibly reducing the impact of both wave scour and environmental fluctuations on fouling organisms. In short, our data suggest that conditions are favorable for the in-transit survival of species on vessels arriving to Alaska from further south.

All five NIS collected alive in this study were first recorded from the coast to the south, and have since been recorded further north. For example, C. mutica was first described on the Pacific coast from California (Marelli 1981) and has since been recorded further north on the coast, including in Alaska from Ketchikan to Dutch Harbor in the northwest (Ashton et al. 2008). B. violaceus and S. japonica, also recorded first in the south, were among those target species found on recreational vessel hulls in British Columbia (Clarke Murray et al. 2011), and were described from multiple bays, including Ketchikan, in recent surveys for marine invasions in Alaska (Ruiz et al. 2006). We also encountered live individuals of the encrusting bryozoan W. subtorquata and the barnacle A. improvisus which had not yet been recorded previously from Alaska. It appears that both of the latter species can tolerate local temperatures in Alaska. Approximate sea surface temperatures for the West Coast states range from 6 to 14 °C in Ketchikan, AK; 9–13 °C in Seattle, WA; 9–13 °C in Newport, OR; 11–16 °C in San Francisco, CA; and 15–26 °C in San Diego, CA (temperatures are annual maximum and minimum based on monthly averages; W. subtorquata adult thermal tolerances (12–28 °C; Cohen 2005) suggest that this species would be able to survive during the Ketchikan summers, although persistent adult and larval survival may be inhibited (larvae tolerate 18–28 °C; Cohen 2005), and niche models suggest that A. improvisus can colonize southeast Alaska (deRivera et al. 2011).

In addition to the above five NIS, the bryozoan C. brunnea is native to the southern northeastern Pacific but has not yet been recorded in Alaska. The thermal tolerance of C. brunnea is not known. Finally, two other NIS found on vessels by Clarke Murray et al. (2011) were found dead on vessels in this study (B. schlosseri and D. listerianum). Our results indicate that small boats may have been responsible for the historic introduction of these biofouling NIS that are now established in Ketchikan and are a possible vector for the future introductions of B. schlosseri, C. brunnea, D. listerianum, and W. subtorquata to the area.

Boater interviews in Ketchikan and previous work highlight that the hulls of transient vessels and frequently used vessels are generally well maintained prior to voyages (Davidson et al. 2008; Floerl and Inglis 2005). Fouling extent has been found to increase with paint age (Floerl et al. 2005a), suggesting that transient vessels arriving to Ketchikan may be relatively clean on arrival. However, we did not find significant correlations between the date of last hull cleaning and extent of fouling cover observed during in-water surveys. The distribution of biofouling species across submerged locations of vessels reinforces the importance of niche areas for fouling communities, noted previously for commercial vessels (Coutts and Taylor 2004). Certain underwater surfaces that act as refugia for biofouling organisms are often overlooked during maintenance, including dock block areas, cooling pipes, and inaccessible niche areas (Coutts and Taylor 2004; Davidson et al. 2009). Furthermore, there is evidence that trace fragments of marine organisms left after cleaning may act as positive settlement cues for marine larvae leading to higher biofouling accumulation (Floerl et al. 2005b). Thus, the quality of the hull-husbandry effort may be as important as frequency.

It is also important to note that there may be a seasonal aspect to hull husbandry and transit that is not captured by our dataset. Many of the boaters we interviewed reported hull cleaning after winter, just prior to their transit to Ketchikan. All of our vessels were sampled in May and June, during the first half of the seasonal pulse (Fig. 2). For arrivals later in the season, increased time since painting and localized boating in southern locations (e.g., Puget Sound) may result in a significantly higher biofouling extent, including species numbers and abundance, on vessels arriving to southeast Alaska. This may depend in part on whether time-of-year (after winter lay-up) or preparation for long-distance travel is the dominant motivation for private boaters to clean their hulls.

The seasonal northward pulse of small vessels and their cumulative biofouling species represent an active mechanism for the introduction and establishment of NIS at the northern edge of a marine bioinvasion front in the northeastern Pacific. There is a large pool of NIS established in the south with great potential for northward range expansion, and ecological niche models indicate that several species already introduced to the south are capable of establishing populations in Alaska (deRivera et al. 2011; Kelley et al. 2013). Although the comparably low richness of NIS found in this study and historically in Alaska suggest that the rate of expansion has been relatively slow, this may also be changing. Vessel traffic and ocean warming may interact to increase the probability of invasions from the south (Ruiz and Hewitt 2009). The continued northward movement of NIS means that vessels will be exposed to an increasing pool of species over time, especially in Washington and British Columbia. Importantly, both recreational and commercial vessel traffic may contribute to this general pattern of spread, but a major expansion of the commercial port at Prince Rupert is also expected to increase the volume of international shipping here (Fan et al. 2009). If this results in increased NIS locally, it may further increase the opportunity for stepwise transfer to Alaska, given that 60 % of small vessels report Prince Rupert as a port of call en route to Ketchikan. In addition, environmental match is predicted to increase with climate change (deRivera et al. 2011).

As a gateway for small vessel traffic to Alaska, Ketchikan is potentially a critical point of entry for the establishment and subsequent spread of nonnative biofouling organisms in Alaska. Any effective management strategy to minimize nonnative biofouling introductions to Alaska should consider Ketchikan as an important model, both to understand species transfer and invasion dynamics but also as a nexus for education and outreach to boat owners. More broadly, it would be worth exploring whether such seasonal and directional pulses of small vessel traffic apply to other continents or global regions and under what circumstances. Such information could be especially valuable in advancing management strategies or establishing sentinel sites for detection of new NIS incursions and response.


This research was funded by the Alaska Department of Fish and Game (project leader Tammy Davis). Gary Freitag, Barbara Morgan, Steve Corporon, Monaca Noble, and Trevor Ruiz provided local assistance in Ketchikan, and we also thank Dr Ernie Meloche for inspiration in the field. Other parties who offered assistance in the form of data, access, local knowledge, or personnel include Allen and Saunya Alloway at Wind and Water Charters and Scuba, Alaska Forestry Service, Ketchikan Customs and Border Protection, and Ketchikan Ports and Harbors. Taxonomic experts were Kristen Larson (tunicates), Jeff Cordell (mobile crustacea), Chris Brown (barnacles), and Linda McCann (bryozoans). The genetic analyses were completed by Jonathon Geller’s laboratory.

Supplementary material

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© Coastal and Estuarine Research Federation (outside the USA) 2014